1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and a semiconductor device, and more particularly, to a semiconductor device with a deep well and a manufacturing method therefor.
2. Description of the Related Art
Semiconductor technology has been unceasingly advancing toward microminiaturization. In the case of transistors, wiring, etc., microminiaturization has evolved according to the scaling law, and performance has also been enhanced by forming, for example, wells (hereinafter referred to as deep wells) with a greater depth than conventional P- or N-type wells to lower power consumption and to remove interference noise.
Microminiaturization of such elements, however, has brought up a number of problems. For example, the problem described below arises during the process of fabrication of a MOSFET (Metal-Oxide Semiconductor Field Effect Transistor).
FIG. 22 is a schematic diagram illustrating ion implantation for the formation of a well. First, a photoresist (not shown) is applied onto a substrate 310, and then a photoresist pattern 320 is formed by exposure and development. Where the exposure is performed using i-line radiation at this time, the border of the photoresist pattern 320 becomes tapered at a certain angle, as shown in FIG. 22. Thus, during the subsequent ion implantation, ions 340 are implanted into regions shallower than a well 330, which is a target doping region with a certain depth, in the vicinity of the border of the photoresist pattern 320.
Meanwhile, with the advance of semiconductor technology toward microminiaturization, exposure techniques using krypton fluoride (KrF) excimer laser light which is shorter in wavelength than i-line radiation have been proposed (e.g., K. Tomita, “Sub-1 μm2 High Density Embedded SRAM Technologies for 100 nm Generation SOC and Beyond”, 2002 Symposium on VLSI Technology, Digest of Technical Papers, pp. 14-15).
FIG. 23 is a schematic diagram illustrating ion implantation for the formation of a well. As in the case shown in FIG. 22, a photoresist is applied onto a substrate 310, and then a photoresist pattern 420 is formed by exposure and development. Where the exposure is performed using KrF excimer laser light at this time, the taper angle can be made smaller than in the case shown in FIG. 22, thus making it possible to reduce the number of ions 440 in the shallow region in the vicinity of the border of the photoresist pattern 420. It is therefore possible to significantly lessen the influence of the ions upon the channel characteristics of a MOSFET which is formed in the subsequent step, thereby enabling microminiaturization of semiconductor devices.
However, the exposure using KrF excimer laser light is associated with the following problem.
FIG. 24 is a schematic sectional view showing a conventional MOSFET with a triple well structure including a deep well. The MOSFET has a P-type impurity-doped region (hereinafter referred to as P-well) 520, an N-type impurity-doped region (hereinafter referred to as N-well) 530, and STIs (Shallow Trench Isolations) 540. Further, under the P-well 520, a deep, N-type impurity-doped region (hereinafter referred to as deep N-well) 510 is formed, thus constituting a triple well structure. The N-well 530 has P-type source/drain regions 550 formed therein, while the gates are not shown in the figure. To form the deep N-well 510, it is necessary that ions should be implanted deeply. In this case, the ions directed to regions other than the region where the deep N-well 510 is to be formed must be captured by the photoresist which serves as a mask. Namely, the photoresist needs to have a thickness large enough to capture the ions.
However, KrF excimer laser light, which is capable of reducing the taper angle, is short in wavelength and small in focal depth. Thus, where the thickness of the photoresist is 1 μm or more, it is difficult to carry out satisfactory exposure. Consequently, i-line radiation longer in wavelength than KrF excimer laser light must be used to ensure satisfactory exposure while at the same time using a photoresist with an adequate thickness.
With the exposure using i-line radiation, however, ions 560 are implanted into a shallow region of the substrate (in the illustrated example, N-well 530) in the vicinity of the border of the photoresist pattern which is used at the time of forming the deep N-well 510, as mentioned above, with the result that the characteristics of the MOSFET are adversely affected. In the case of the structure as shown in FIG. 24, therefore, the MOSFET needs to be formed at a location free from the influence of the ions 560, which, however, leads to increase of the chip area.